(1) Field of the Invention
This invention relates to resolving emulsions by electric field treatment and apparatus therefor.
As used herein, the term "emulsion" is used to include dispersions resembling emulsions as well as true emulsions. The emulsions to which this invention relates may be categorized as formed of immiscible external and internal liquid phases, the internal phase being an aqueous material and the external phase an organic material. The internal aqueous phase has a higher dielectric constant and conductivity than the external organic phase.
The aqueous material may contain various water-soluble impurities such as chloride ions, and non-soluble salts or inorganic solids, such as sand, entrained therein.
The emulsion subjected to electrical treatment for its resolution may already exist as a stream from a natural source, or a stream associated with a production facility such as an oil refinery or a plant arranged for production of synthetic chemicals or other materials. However, the emulsion may be formed artificially, or further altered, by mixing an aqueous medium with the liquid organic material.
This invention relates more specifically to the resolution of emulsions formed between water and highly conductive, high viscosity and low API gravity crude oils.
(2) Description of the Prior Art
Electric fields are employed for resolving many emulsions in which the internal phase is an aqueous material such as water, caustic, or acid, etc., and the external phase is an organic liquid material such as crude oil. These emulsions are passed between electrodes energized with a high voltage to create an electric field that causes the internal phase to coalesce. The term "coalesce" as used herein refers to the agglomeration of the dispersed internal phase while in the continuous external phase. Sufficiently large particle sizes of the internal phase are created which can be readily separated from the external phase by differences in specific gravities.
Conventional "electric field" techniques for resolving water and crude oil emulsions require an electric field of certain potential gradient magnitudes for the electric treatment of the emulsion to be considered practical. For example, the high voltage applied to the electrodes in conventional treaters is generally between about 11,000 volts and about 33,000 volts, or even higher. Usually, the electrodes are spaced apart from about 4 to about 11 inches. Thus, conventional practices generally produce treating potential gradients from about 2.5 kv to about 8.5 kv per inch spacing between electrodes. This electrode-spacing, high voltage criterion of potential gradient exists whether all electrodes are energized or whether some electrodes are energized and others are grounded.
In carrying out these conventional electric treatments fro resolving emulsions, a variety of treaters were developed. These are two main types, which may be termed high velocity and low velocity treaters, respectively. Examples of high velocity treaters are those described in the following U.S. Pat. Nos. 2,443,646, 2,527,690, 2,543,996, 2,557,847, 2,880,158, and 2,894,895. Examples of low velocity treaters are those described in the following U.S. Pat. Nos. 2,033,129, 2,033,137, 2,098,982, 2,102,051, 3,396,100, 3,458,429, 3,369,500 and 3,672,511. Among other U.S. patents describing electric treaters of the conventional type are: U.S. Pat. Nos. 2,182,145, 2,855,356, 2,976,228, 3,205,160 and 3,205,161.
Treaters of conventional design have given satisfactory service in most applications. However, in a number of situations, especially where more conductive, viscous and low API gravity crude oils are encountered, the electrical power consumption of the treater is excessive and erratic. In some treater applications, the electrical stability is severely affected by large current flows that cause the protective devices on the treater to remove, or reduce, operating potentials from the energized electrode(s). Where treating did occur, the efficiency of emulsion resolution in these electric treaters was adversely affected because the electric treatment was not maximized. More importantly, a change in voltage, flow rate, temperature or other operating parameter, that corrects a certain problem with a treater resolving one emulsion is totally inadequate to remedy the same problem with a different emulsion. This phenomena has been observed for almost 50 years in the electrical resolution of emulsions. In summary, it may be stated that identical electrical treater configurations react differently and also erratically (non-linearly), upon resolving emulsions having varying properties including relative amounts of internal and external phases, their chemical compositions, and the conductivity of the continuous phase.
Many reasons have been given for the unpredictable operation of the treaters with such emulsions. For example, a substantial quantity of dispersed water is collected between the electrodes in these treaters. Additionally, masses of water are collected between the edges of the energized electrode and the metal walls of the containing vessel. These masses of water result from hydraulics-flow patterns within the treater and/or the electric field effects upon the emulsion. The masses of water can align themselves to form highly conductive paths to electrical current. This phenomena is commonly termed "chaining". Chaining leads to the conduction of very heavy current flows between an energized electrode and adjacent grounded electrodes or metallic walls of the treater. These heavy currents produce an excessive loading upon the power transformer. The protective reactance decreases the voltage applied to the primary of the transformer which automatically results in a substantial decrease in the energization potential applied to these electrodes. As a result, the trapped water falls, thereby alleviating the chaining condition, but reducing the potential to a point below that required for coalescence. Many of these treaters are in "balanced" operation with a potential applied to the energized electrode which treats the emulsion at some reduced flow conditions to avoid encountering the chaining phenomena. A slight change in any operating condition makes the treater incapable of resolving the emulsion, and/or produces chaining which cause a substantial reduction in treating gradient in the electric field and resolution of the emulsion.
The energized and grounded electrodes can be spaced relatively far from one another or adjacent grounds, to avoid "chaining". Although avoiding chaining effects, the operation of the treater still is not always predictable. For example, it has been urged that the higher the potential applied to the energized electrode(s), the better the treater would resolve the emulsion subjected to it. This theory was not always found to be correct since an increase in potential which energized the electrodes in many instances would cause a relatively greater increase in the current drawn from the secondary of the transformer. This heavy demand would cause the protective reactance to decrease immediately the potential applied to the energized electrode(s). As a result, treating efficiency would not be improved even at an increase in power applied and consumed.
The high velocity design attempts to eliminate "chaining" hydraulically by using a velocity selected to keep the emulsion turbulent as it is injected into the field; the low velocity design by slowing the charge rate such that water falls from the electric field before "chaining" takes place.
Many types of emulsions, particularly the low API gravity, high viscosity crude oils, make the operation of conventional treaters very sensitive to adjustments in its operating parameters. For example, small changes in any of the operating conditions in the treater cause sudden and great changes in the resolution of emulsion. Furthermore, these changes are not predictable or uniform in magnitude. An upset in operation of the treater usually produces relatively large changes in the electrical field, and correspondingly, each of the energized electrode(s) increasingly consumes current from the power transformer.
The unpredictability of changes in the electrical system and their effect upon emulsion treating efficiency in conventional treaters has led to excessive power consumption and reduced treating efficiency under certain treating conditions. For example, emulsions formed of high viscosity, low API gravity crude oils, such as those produced by steam flooding or fire flooding procedures, are typical examples of the problem area. The conventional treaters are difficult to place into operation, and to operate, with these crude oils.
Moreover, in the prior art treaters described in the above listed patents, the treatment is dependent on the settling of the coalesced aqueous material drops. That is, these treaters are, in effect, electrically aided settlers. The settling velocity of the coalesced drop is therefore a limitation on the efficiency of the treater. This settling velocity obeys Stokes' law for water droplet settling, which may be expressed: ##EQU1## where: V is the settling or terminal velocity of the falling water droplet;
r is the radius of the drop (assumed to be a sphere); PA1 v is the viscosity of the oil through which the water is flowing; PA1 P.sub.p is the density of the falling drop; PA1 P.sub.h is the density of the oil through which the water is falling; and PA1 k is a suitable constant for the system being resolved.